This invention relates to relief valves, and more particularly, to relief valves which alternatively relieve either excessive pressure or excessive vacuum within a closed fluid storage tank.
Relief valves of this general type have been developed for installation on both stationary and transportable fluid storage tanks. Such tanks generally contain fuels or other volatile liquids which can produce excessive pressure within the tank caused by ambient temperature or product agitation. In addition, these relief valves have been designed also to relieve a reduced pressure or developing vacuum within the tank resulting from cooling of the fluid product. Examples of such relief valves are described in U.S. Pat. Nos. 3,974,850 and 3,580,275.
In fluid storage tanks employed for transport by truck, rail or ship, the danger from leakage of fluid when the tank becomes tilted excessively or overturned has lead to development of relief valves which will seal in the event of such occurrence. U.S. Pat. No. 3,913,601 describes a relief valve in which the opening to the atmosphere can be closed by a check valve operated by gravity, but this valve only operates when it is completely inverted. U.S. Pat. No. 4,287,910 describes a spring-loaded relief valve designed to maintain a closed position should the storage tank be overturned. However, this valve could open so as to allow undesirable leakage of vapor under normal pressure conditions and upright orientation of the storage tank in transport which is subjected to jolting and bumping. Under these adverse conditions, the inertia of a weighted valve member can compress the biasing spring and thereby, permit the valve to open with resulting leakage of tank contents.
It is believed that these prior patents do not provide a valve mechanism which successfully prevents leakage of tank contents under adverse conditions of transport of the tank and adverse tilting or roll-over of the tank.